Liquid surface activity is related to liquid surface tension. As surface activity increases, surface tension decreases. Therefore, measuring relative liquid surface tension enables the detection of liquid surface activity.
"Dynamic surface tension" refers to the apparent surface tension of a fluid surface in which the surface activity has not reached equilibrium. A drop which is formed infinitely slowly would give the equilibrium value of surface tension--this is the classical definition of surface tension. A drop which was growing only five seconds has a "dynamic" surface tension if its surface activity has not reached equilibrium. For some solutes which quickly equilibrate, such as ethanol, dynamic effects would not appear until drop intervals well under 0.1 seconds are achieved. However, some surface active proteins, such as those found in beer, can literally take hours to equilibrate. Practically speaking, it is excruciatingly difficult to measure their "equilibrium" surface tension, by any means. However, sufficient effects occur at such surfaces after only seconds so that dynamic surface tension can be measured to detect surface activity.
There are many techniques for the measurement of liquid surface tension. The DuNouy ring, Wilhelmy plate, capillary rise, maximum bubble pressure, and drop weight or drop volume methods are some of the most popular techniques. These techniques are generally employed on pure liquids and/or some solutions for equilibrium surface tension measurements. In addition, they can also be used to follow relatively slow dynamic or non-equilibrium changes in surface tension.
However, most of the prior art techniques are not amenable to total automation in order to monitor the surface tension, and therefore the surface activity, of a liquid having a continuously and relatively rapidly changing solute concentration. This need may arise, for example, in the monitoring of industrial effluents, chemical processing, reaction progress, or chromatographic eluate.
For example, in liquid chromatography, a mixture of substances are separated based upon the physical or chemical nature of the substances. A solution of the mixture is applied to the top of a chromatography column or injected into the eluent (a flushing liquid) which is pumped at constant volume flow over a usually solid or gel-like material called the chromatography packing.
The packing material is usually encased in a glass or stainless steel cylinder called the chromatography column. The packing material interaction with a particular component of the applied mixture determines the rate which the component travels the length of the column. For example, a packing material which operates by a "size-exclusion" mechanism has a generally porous structure; small molecules are trapped within these pores. The smaller molecules are thus slowed in their travel of the length of the column. Consequently, large molecules will exit in the column eluate (the liquid exiting the column) well before molecules which are smaller than the pore size of the packing.
Other separations are based upon chemical or physical attraction to the packing material such as in ion-exchange chromatography. Such packing material can completely prevent passage of particular classes of chemical structure allowing only non-binding compounds to pass unhindered. The bound substances can be released by means of a specially prepared eluent.
If an eluting component is surface-active, it will change the surface activity of the eluate from the chromatography column. Therefore, monitoring the surface activity of the eluate yields information on the presence or nonpresence of the component. Automatically providing this information, quickly, and in some applications with only a very small sample, has not been available in the prior art.
Only one of the surface tension techniques mentioned above, maximum bubble pressure, appears to have been adapted to a completely automated instrument as disclosed in the Klus et al. U.S. Pat. No. 4,416,148, issued Nov. 22, 1983. However, it cannot be employed for measuring small (less than 1 mL) liquid sample volumes. Therefore, it would be desirable to have a method and apparatus for measuring the relative liquid dynamic surface tension of small sample volumes in an automated and continuous manner.